A polyester polymer such as intermolecularly cyclized ester, e.g., dilactide and diglycolide, monomolecularly cyclized ester, e.g., lactone, and copolymer thereof (hereinafter simply referred to as "polyester polymer") undergoes decomposition by light, heat, oxygen or the like to enter into the natural reduction cycle. Thus, it has recently been the target of studies for use as a biodegradable polymer material from the standpoint of safety and prevention of global environmental pollution.
The process for the production of a homopolymer of dilactide or diglycolide as an intermolecularly cyclized eater can be roughly divided into two known groups.
One of the two processes comprises direct dehydropolycondensation of the corresponding hydroxycarboxylic acid to obtain a polymer. The other comprises the synthesis of a dehydrated cyclic ester of hydroxy acid known as an example of dilactide or diglycolide, and then the ring-opening polymerization of the ester to obtain a polymer.
In the former direct polycondensation process, it is difficult to obtain a polymer having a molecular weight of not less than 4,000 (as described in C. H. Halten, "Lactic Acid", page 226, Verlag Chemie, 1971). Even if the reaction conditions are improved in an attempt to increase the high molecular amount of the polymer, the limit of the molecular weight is about 20,000 as described in JP-B-2-52930 (The term "JP-B" as used herein means an "examined Japanese patent publication"). If a polymer having a higher molecular weight is required to be prepared, the latter ring-opening polymerization of cyclic esterified product has been heretofore used.
Referring to the process for the continuous production of these lactides or lactones, processes for the continuous production of aromatic polyesters and lactones are disclosed in JP-A-61-281124 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-61-283619, JP-A-61-287922, JP-A-62-20525, JP-A-60-27425, JP-A-2-302428, JP-A-2-302429, JP-A-2-302433 and JP-A-2-302434, In these processes, the reaction vessel comprises screw or paddle agitating blades as used in kneader or extruder incorporated therein. In this arrangement, the reaction system is agitated by the agitator while the reactants are sequentially moved from the intake port to the product outlet.
Referring to the process for the continuous production of lactides, JP-A-5-93050 discloses a so-called CSTR continuous production process which comprises continuously supplying starting materials into a series combination of a plurality of agitating tanks so that they are subjected to continuous polymerization for a retention time between the first reaction tank and the final reaction tank as reaction time. However, all the above cited processes are operated by reaction apparatus equipped with dynamic agitators, and the foregoing patents give no disclosure or suggestion of solution to difficulty in uniform agitation and heat removal caused by the rise in the viscosity of the reactants in the continuous production of a biodegradable polyester polymer having a high molecular weight from lactides or lactones.
In some detail, if the processes for the production of lactides as disclosed in the above cited patents and references are tried, it can be found that as the average molecular weight of the polymer thus produced rises, the polymer viscosity rises to as very high as 10,000 to several hundreds of thousands of poise, making it difficult to agitate the reactants by an ordinary agitator, even to withdraw the reactants. Even if a powerful agitator is used and an elaborate agitating blade is used to agitate the reaction system, the reactants move only in a substantially laminar flow according to the rotation of the agitating blade, making it difficult to homogeneously agitate the entire reaction system.
Further, since the ring-opening polymerization of cyclic esters involves heat generation, the resulting rise in the viscosity of the reactants gives difficulty in uniform agitation that makes it difficult to control the temperature in the reaction tank. This makes the reaction out of control or gives a distribution of temperature in the polymer, causing local heating that deteriorates the polymer quality.
In particular, these biodegradable polyester polymers prepared from cyclic esters have an excellent biodegradability but easily undergo hydrolysis by acid, alkali or water and liable to drop of molecular weight due to heat. For example, GUPTA M. C., "Colloid Polymer Science" DEU, 260 (3), 308-311, 1982, reports studies of the thermal decomposition rate of a homopolymer of dilactide by TGA (thermogravimetric analysis). However, an accelerated drop of molecular weight takes place at a temperature as high as not lower than 250.degree. C. even in a sealed reaction vessel.
In addition, the homopolymer or copolymer of dilactide is also liable to coloring upon exposure to high temperature. That is, the conventional process for the continuous production of polymers from cyclic esters is disadvantageous in that the rise in the viscosity of the polymer caused by the rise in the molecular weight of the polymer prevents uniform mixing, resulting in local heating that causes partial modification and hence quality drop. Accordingly, setting aside small scale laboratory experiments, an improved process for the mass production of a biodegradable polyester polymer on an industrial basis has been desired.